Onboard ensemble timescale realization for synchronization of future G2G-like constellations

Kurzfassung

Next generation Global Navigation Satellite Systems (GNSSs) will most likely benefit from inter-satellite links (ISLs). ISLs enable the generation of ranging observables between satellites, as well as performing time transfer, and they provide a data relay function across the GNSS constellation. Thanks to these capabilities, a paradigm shift is possible for the autonomous realization of a system timescale. Instead of a ground-based realization of the timescale, where each satellite clock offset is estimated, an onboard realization of the timescale and estimation of the clock offset is enabled by the relay of time offset measurements obtained via ISL. These are processed onboard each satellite by an algorithm establishing a distributed clock ensemble, composed of all the satellites in the constellation. In this way, each onboard clock can be directly aligned to the constellation system time. Assuming a real-time distribution of the ISL observables across the constellation, an autonomous synchronization of all satellites can be achieved.

This work analyzes the feasibility of such a scheme in a Galileo 2nd Generation (G2G) scenario and presents the achievable synchronization level. G2G will be equipped with radio-frequency ISLs establishing a dual one-way data exchange between pairs of satellites. After the data exchange, the link topology is changed to connect to a different satellite. This design introduces some constraints on the proposed onboard synchronization scheme: firstly, compared to a twoway time transfer, the time division of the G2G link impacts the achievable time transfer accuracy by limiting the synchronism of the time exchanges. Secondly, the real-time distribution of observables is not guaranteed, since only one link per satellite is established at any epoch. Instead, the data is distributed in a multi-hop fashion using the "flooding by broadcast" approach. Thus, each satellite only receives a subset of measurements with different delays. Processing these measurements on board results in different local computations of the system timescale, thus degrading the satellite synchronization.

In this work, we firstly generate realistic G2G time offset observables by: 1) estimating the achievable performance of the time transfer in a G2G scenario assuming the G2G data exchange design and predicted orbit information; 2) simulating the distribution of the observables using a dedicated scheduling algorithm considering the G2G link constraints; and 3) generating time offset observables using simulated G2G clocks or equivalent measurements of real atomic clocks in a laboratory.

Then, with this data we assess the performance of a real-time onboard synchronization scheme. The scheme makes use of a local computation of a distributed clock ensemble based on Kalman filtering using the time offset measurements locally received. The performance of the proposed method is evaluated in terms of achievable real-time synchronization error between satellites, continuity of the generated timescale, as well as computational and data transfer needs. Finally, the performance is compared to a benchmark scenario, implementing a batch Kalman filter making use of all the available measurements. This last case represents a post-processed, groundbased timescale generation using all the collected inter-satellite observables.